Belt-type casting machines have been provided heretofore and can include a pair of synchronously moving metal belts passing over driven and/or undriven rollers or drums and which define between them a gap into which molten metal is poured for progressive solidification as the casting is drawn by the belt from the liquidus zone. The belts are cooled by water or air and, downstream of the liquidus zone, a solidified casting is recovered. The belts generally lie at an inclination to the horizontal and the space in which the molten metal solidifies may be confined between a pair of lateral stationary or movable walls.
In order to obtain a satisfactory continuously cast strip, the codirectionally moving stretches of the belt (i.e. the upper and lower passes) which define the solidification space in the longitudinal and transverse directions, must form exactly planar surfaces. Such planarity can be readily achieved by making the belt very thick, but this has the disadvantage that the flexing ability of the belt is reduced and, as the belt repeatedly passes over the rollers and is subjected to cyclic bending stresses, deterioration of the belt occurs. Accordingly, the practice has been to provide casting belts of high-strength material under high tension.
Another requisite of the conventional belt-type casting system is that the belts must serve not only to impart the desired shape to the solidifying melt, but must also dissipate the latent heat of fusion so that solidification can occur. For this purpose the belts may be sprayed with a liquid coolant at their surfaces turned away from the continuous casting.
Because one side of the belt may be subjected to the coolant temperature while the other side is subjected to the melt temperature, the belts of conventional system tend to undergo differential thermal expansion such that adequate tensioning of the belts cannot be guaranteed.
It will be apparent that a continuous casting apparatus will have value only if the wear of the belt is minimized and need not require frequent replacement. It has already been proposed to provide a steel belt for a continuous casting system with a heat-lagging coating or thermally insulating layer so that a thermal resistance is provided across the belt and wear of the latter is reduced. However, since in prior systems the coating was abraded by the casting metal, the belts were not entirely satisfactory. The coatings must not only be temperature-resistant (refractory), heat-lagging (thermally insulating), flexible and tough, they must also adhere preferentially to the substrate metal of the belt rather than to the casting metal and must have a high tearing strength.
Some of the coatings which have been provided heretofore are dispersions of finely divided refractory material, such as diatomaceous earths, to which finely divided carbon, for example, graphite, is added. The dispersion is provided in an aqueous or alcohol solution containing solubilized organic binder, the solvent being evaporated after the dispersion has been coated onto the belt. To improve adherence to the belt, the substrate metal is roughened by sandblasting with corundum or by steel-shot peening. This insures a mechanical interlock between the coating and the steel substrate at the interface.
It has long been recognized in belt-type metal-casting apparatus, that some relative movement between the casting and the belt surface will occur. This relative movement results in abrasive wear of the heat-lagging coating and such coatings, where they do not resist abrasion, must be replaced or renewed frequently at considerable cost.
Another disadvantage of the conventional systems is that abrasive material from the coating frequently becomes entrained by the casting so that particles remain during subsequent rolling processes or leave a surface which does not roll effectively. Accordingly, the rolled products which are made from the continuous casting can have low quality.
There has also been described the application of a pulverulent parting agent such as talc, on an organic coating during operation of the casting belts. The parting agent serves to reduce the abrasive wear of the coating by preventing the casting from adhering to the coating material. The disadvantage of this arrangement is that the parting agent must be continuously removed whereby wear of the coating occurs and at least some particles of the parting agent remain entrained by the cast strip and are firmly locked into its surface.
Systems in which the casting bands are coated with ceramic materials provide an especially effective heat lag or thermal insulation, but these materials have little abrasion resistance since the coating, because of its brittleness, has a tendency to break down into fine particles which contaminate the surface of the cast strip.
When the belt-type casting machine is provided directly upstream of one or more rolling stands so that the residual heat of the casting can be used for the rolling process, the abrasive particles must be removed before the casting encounters the first rolls.
This is extremely difficult because of the high ductility of the continuous casting which limits mechanical removal of the particles. A material-removal technique, as is necessary to eliminate the particles, causes a significant loss of metal with significant economic disadvantages. Finally any attempt to flush the abraded particles from the surface by gaseous or liquid fluids removes only the loose particles and is ineffective against those particles which are firmly embedded in or bonded to the surface.